The present invention relates generally to gasification systems, and more particularly, to methods and apparatus for recycling tail gas to facilitate enhancing synthetic gas production with a gasification system via acid gas removal.
At least some known gasification plants include a gasification system that is integrated with at least one power-producing turbine system, thereby forming an integrated gasification combined cycle (IGCC) power generation plant. For example, known gasification systems convert a mixture of fuel, air or oxygen, steam, and/or CO2 into a synthetic gas, or “syngas”. The syngas is channeled to the combustor of a gas turbine engine, which powers a generator that supplies electrical power to a power grid. Exhaust from at least some known gas turbine engines is supplied to a heat recovery steam generator (HRSG) that generates steam for driving a steam turbine. Power generated by the steam turbine also drives an electrical generator that provides electrical power to the power grid.
At least some known gasification systems associated with IGCC plants initially produce a “raw” syngas fuel that includes carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), carbonyl sulfide (COS), and hydrogen sulfide (H2S). CO2, COS, and H2S are typically referred to as acid gases. Acid gas is generally removed from the raw syngas fuel to produce a “clean” syngas fuel for combustion within the gas turbine engines.
Typically, in known IGCC plants, hydro-carbonaceous feeds are reacted with high purity oxygen (typically 95% oxygen purity) to produce syngas in a temperature range of 2200° F. to 2700° F. The syngas produced is then cooled and scrubbed to produce raw syngas that is suitable for introduction to an acid gas removal (AGR) subsystem. Acid gas removal is performed with the acid gas removal subsystem that typically includes at least one main absorber to remove a majority of H2S and COS. The acid gas removal subsystem absorbs most of the sulfur present (in the form of H2S and COS), as well as a portion of CO2 present. This produces a clean syngas stream and an acid gas stream consisting primarily of the absorbed H2S, COS and CO2.
The acid gas stream is treated in a sulfur reduction subsystem to convert sulfur present into elemental sulfur by the Claus reaction. This step consists of first oxidizing some of the hydrogen sulfide present to form sulfur dioxide, and then forming elemental sulfur by the following reaction:2H2S+SO2→3S+2H20This reaction occurs in several reactors in series. After each reactor, sulfur is removed as a liquid product. The effluent from the sulfur reduction subsystem consists primarily of carbon dioxide, nitrogen and water vapor, along with unreacted hydrogen sulfide and sulfur dioxide. This effluent is called Claus tail gas.
Claus tail gas usually requires further processing to remove sulfur prior to being released. This step is called tail gas treating. The most common process used is called SCOT (Shell Claus off-gas treating). A SCOT process consists of first hydrogenating the Claus tail gas to convert sulfur dioxide into hydrogen sulfide. The hydrogenated gas is then cooled to enable removal of the water vapor as condensate. An amine solvent is used to capture the hydrogen sulfide. Finally, the solvent is regenerated to produce a hydrogen sulfide-containing stream for recycle to the sulfur recovery unit. The non-absorbed off gas is sent to a thermal oxidizer and emitted.
It has now been discovered that it is possible to eliminate the tail gas treating unit by recycling the Claus tail gas to the gasification reactor. Such a process facilitates eliminating the costs, and complications of treating the Claus tail gas. In addition, a reduced cost Claus unit may be used with a lower recovery or, for example, fewer stages of recovery, since the tail gas is recycled to the gasification reactor. As such, the present invention provides a method and apparatus for treating gas produced in a gasification reactor, for example an IGCC system, comprising recycling gas subjected to acid gas removal and sulfur reduction to the gasification unit of the system without intermediate hydrogenation.